The present invention relates to a device for determining the influence of physical parameters such as pressure, ambient temperature, shear or tensile forces, or the like, on the length of a path, with an optical length-measuring device responding to the change in path length, said measuring device being capable of producing an electrical display signal characteristic at least of the change in path length.
It is known to determine changes in physical parameters, which can be visualized as changes in optical path lengths. In many cases, this procedure is equivalent to a change in a mechanical path length, in such a manner that these path length changes are measured with the aid of an interferometer (see, e.g. Luger, Lexikon der Tecknik, Rowohlt, 1972, Vol. 26, p. 479). A device suitable for measuring changes in high temperatures, using a Michelson interferometer, of the type cited hereinabove, can then be so designed, for example, that the movable mirror of the interferometer is mounted at the free end of a rod, said rod being firmly gripped at its other end. The temperature expansion coefficient of the rod is known, so that the changes in the length of the rod provide a direct indication of the temperature changes correlated therewith. The number of output pulses from a receiver located at the interference point, said pulses occurring sequentially in the course of a temperature change from T.sub.1 to T.sub.2, and said receiver generating an output pulse in response to constructive interference, is then a means of directly measuring the temperature. These receiver output pulses can be detected by an electronic counter and displayed appropriately in digital or analog form.
Generally, interferometric measurements are very accurate, since the absolute measurement error is only on the order of magnitude of the wavelength of the coherent light used for measurement. But a device of the type described hereinabove, provided with an interferometer as the measuring device, suffers from serious disadvantages, which render it unsuitable for numerous applications.
For example, in interferometric measuring systems, the measurement range is theoretically limited by the coherence length of the measuring light used. Admittedly, it is quite possible by using lasers to have coherence lengths of several kilometers in the optical range. However, the engineering costs required are considerable, and the coherence length is only in the meter range (for example 1 to 5 meters) when a conventional laser is used as the light source.
In practice, this means that in many instances the changes in length of a path marked by a body, associated with changes in a certain parameter, can be detected very precisely. But this is not so in regard to the reference length of the body subjected to the change in length, associated with a certain value of this parameter. Thus, in the final analysis, the absolute value of the modified parameter suffers from the error with which the reference value can be determined by other measurements, which is generally carried out with less accuracy.
On the other hand, it is practically impossible to detect changes in length which occur along a curved path, since the path of the measuring beam, which requires at least intermittent rectilinear light paths with direct, undisturbed visual contact, cannot be adjusted with sufficient accuracy to any path. This is also true of the case in which a transit-time measuring device is used, as in making absolute measurements of large distances (for example, the distance from the Earth to the Moon), in order to determine the length of a path by measuring the transit time of light pulses between the beginning and end of this path.